IgG subclasses of anti-FVIII antibodies during immune tolerance induction in patients with hemophilia A

High levels of anti-factor VIII immunoglobulin G4 and immunoglobulin G total are associated with immune tolerance induction failure in people with congenital hemophilia A and high-responding inhibitors

Background

Immune tolerance induction (ITI) is the treatment of choice to eradicate neutralizing anti-factor (F)VIII alloantibodies (inhibitors) in people with inherited hemophilia A. However, it is not successful in 10% to 40% of the cases. The biological mechanisms and biomarkers associated with ITI outcome are largely unknown.

Objectives

The aim of this study was to investigate the association of plasma cytokines (interferon-γ, tumor necrosis factor, interleukin [IL]-2, IL-4, IL-5, IL-6, IL-10, and IL-17A), chemokines (IL-8/CXCL8, RANTES/CCL5, MIG/CXCL9, MCP-1/CCL2, and IP-10/CXCL10), and anti-FVIII immunoglobulin (Ig) G total, IgG1, and IgG4 with ITI outcome.

Methods

In this cross-sectional analysis of the Brazilian Immune Tolerance Study, we assessed plasma levels of anti-FVIII IgGs using an enzyme-linked immunosorbent assay with plasma-derived FVIII and recombinant FVIII as target antigens, immobilized in microplates.

Results

We assayed 98 plasma samples of moderately severe and severe (FVIII activity, <2%) people with hemophilia A after completion of a first ITI course. Levels of anti-recombinant FVIII IgG total and IgG4 were higher in people with hemophilia A who failed ITI (IgG total optical density [OD], 0.37; IQR, 0.15-0.73; IgG4 OD, 2.19; IQR, 0.80-2.52) than in those who had partial (IgG total OD, 0.03; IQR, 0.00-0.14; IgG4 OD, 0.39; IQR, 0.09-1.11; P < .0001 for both) or complete success (IgG total OD, 0.04; IQR, 0.00-0.07; IgG4 OD, 0.07; IQR, 0.06-0.40; P < .0001 for both). Plasma cytokines, chemokines, and anti-FVIII IgG1 were not associated with ITI outcome.

Conclusion

Our results show that high levels of plasma anti-FVIII IgG4 and IgG total are associated with ITI failure.

The spectrum of neutralizing and non-neutralizing anti-FVIII antibodies in a nationwide cohort of 788 persons with hemophilia A

Objectives

Anti-factor VIII (FVIII) antibodies have been reported to exhibit both neutralizing and non-neutralizing characteristics. This is the first study investigating the full spectrum of FVIII-specific antibodies, including non-neutralizing antibodies, very-low titer inhibitors, and inhibitors, in a large nationwide population of persons with hemophilia A of all severities.

Methods

All persons with hemophilia A (mild (FVIII > 5-40 IU/dL)/moderate [FVIII 1-5 IU/dL)/severe (FVIII < 1 IU/dL)] with an available plasma sample who participated in the sixth Hemophilia in the Netherlands study between 2018 and 2019 were included. The presence of anti-FVIII antibodies of the immunoglobulin A, M, and G isotypes and IgG subclasses, along with antibody titer levels, were assessed using direct-binding ELISAs. FVIII specificity was assessed using a competition-based ELISA approach. The inhibitor status was determined using the Nijmegen ultra-sensitive Bethesda assay (NusBA) and the Nijmegen Bethesda assay (NBA).

Results

In total, 788 persons with hemophilia A (336 (42.6%) mild, 123 (15.6%) moderate, 329 (41.8%) severe hemophilia) were included. The median age was 45 years (IQR 24-60), and the majority (50.9%) had over 150 exposure days to FVIII concentrates. Within our population, 144 (18.3%) individuals had non-neutralizing FVIII-specific antibodies, 10 (1.3%) had very low-titer inhibitors (NusBA positive; NBA negative), and 13 (1.6%) had inhibitors (both NusBA and NBA positive). IgG1 was the most abundant FVIII-specific antibody subclass, and the highest titer levels were found for IgG4. In individuals without a reported history of inhibitor development, no clear differences were observed in antibody patterns between those who were minimally or highly exposed to FVIII concentrates. IgG4 subclass antibodies were only observed in persons with a reported history of FVIII inhibitor or in those with a currently detected (very low-titer) inhibitor.

Conclusion

In this cross-sectional study, we identified non-neutralizing antibodies in a relatively large proportion of persons with hemophilia A. In contrast, in our population, consisting of persons highly exposed to FVIII concentrates, (very low-titer) inhibitors were detected only in a small proportion of persons, reflecting a well-tolerized population. Hence, our findings suggest that only a small subpopulation of non-neutralizing FVIII-specific antibodies is associated with clinically relevant inhibitors.

The unique properties of IgG4 and its roles in health and disease

IgG4 is the least abundant subclass of IgG in human serum and has unique functional features. IgG4 is largely unable to activate antibody-dependent immune effector responses and, furthermore, undergoes Fab (fragment antigen binding)-arm exchange, rendering it bispecific for antigen binding and functionally monovalent. These properties of IgG4 have a blocking effect, either on the immune response or on the target protein of IgG4. In this Review, we discuss the unique structural characteristics of IgG4 and how these contribute to its roles in health and disease. We highlight how, depending on the setting, IgG4 responses can be beneficial (for example, in responses to allergens or parasites) or detrimental (for example, in autoimmune diseases, in antitumour responses and in anti-biologic responses). The development of novel models for studying IgG4 (patho)physiology and understanding how IgG4 responses are regulated could offer insights into novel treatment strategies for these IgG4-associated disease settings.

Anti-FVIII antibodies in Black and White hemophilia A subjects: do F8 haplotypes play a role?

The most common complication in hemophilia A (HA) treatment, affecting 25% to 30% of patients with severe HA, is the development of alloimmune inhibitors that foreclose the ability of infused factor VIII (FVIII) to participate in coagulation. Inhibitors confer significant pathology on affected individuals and present major complexities in their management. Inhibitors are more common in African American patients, and it has been hypothesized that this is a consequence of haplotype (H)-treatment product mismatch. F8 haplotypes H1 to H5 are defined by nonsynonymous single-nucleotide polymorphisms encoding sequence variations at FVIII residues 1241, 2238, and 484. Haplotypes H2 to H5 are more prevalent in individuals with Black African ancestry, whereas 80% to 90% of the White population has the H1 haplotype. This study used an established multiplex fluorescence immunoassay to determine anti-FVIII antibody titers in plasma from 394 individuals with HA (188 Black, 206 White), measuring their binding to recombinant full-length H1 and H2 and B-domain-deleted (BDD) H1/H2, H3/H5, and H4 FVIII proteins. Inhibitor titers were determined using a chromogenic assay and linear B-cell epitopes characterized using peptide microarrays. FVIII-reactive antibodies were readily detected in most individuals with HA, with higher titers in those with a current inhibitor, as expected. Neither total nor inhibitory antibody titers correlated with F8 haplotype mismatches, and peptides with D1241E and M2238V polymorphisms did not comprise linear B-cell epitopes. Interestingly, compared with the full-length FVIII products, the BDD-FVIII proteins were markedly more reactive with plasma antibodies. The stronger immunoreactivity of BDD-FVIII suggests that B-domain removal might expose novel B-cell epitopes, perhaps through conformational rearrangements of FVIII domains.

Determinants of successful immune tolerance induction in hemophilia A: systematic review and meta-analysis

Background

Immune tolerance induction (ITI) aims to eradicate anti-factor VIII (FVIII) antibodies (inhibitors) in persons with hemophilia A. However, this burdensome treatment fails in 10% to 40%. To estimate the chance of ITI success in clinical decision making, it is important to identify the predictors of ITI success.

Objectives

We performed a systematic review and meta-analysis to summarize the current evidence on determinants of ITI outcome in persons with hemophilia A.

Methods

A literature search was conducted to identify randomized controlled trials, cohort, or case-control studies reporting on the predictors for ITI outcome in persons with hemophilia A. The main outcome was ITI success. Methodological quality was assessed using an adapted Joanna Briggs Institute checklist, rating as high if ≥11 of 13 criteria were met. Pooled odds ratios (ORs) for ITI success were calculated for each determinant. ITI success was defined as negative inhibitor titer (<0.6 BU/mL), FVIII recovery ≥66% of expected, and FVIII half-life ≥6 hours in 16 (59.3%) studies.

Results

We included 27 studies, involving 1,734 participants. Methodological quality of 6 (22.2%) studies (418 participants) was rated as high. Twenty different determinants were assessed. Historical peak titer ≤100 BU/mL (compared with >100 BU/mL, OR, 1.7; 95% CI, 1.4-2.1), pre-ITI titer ≤10 BU/mL (compared with >10 BU/mL, OR, 1.8; 95% CI, 1.4-2.3), and peak titer during ITI ≤100 BU/mL (compared with >100 BU/mL, OR, 2.7; 95% CI, 1.9-3.8) were associated with a higher chance of ITI success.

Conclusion

Our results suggest that determinants related to the inhibitor titer are associated with ITI success.

Longitudinal profiling of anti-factor VIII antibodies in Japanese patients with congenital hemophilia A during factor VIII replacement and immune-tolerance induction therapy

When patients with hemophilia A develop factor VIII (FVIII) inhibitors, FVIII replacement therapy becomes ineffective. Although immune-tolerance induction (ITI) therapy has been used to eradicate inhibitors, treatment is unsuccessful in approximately 30% of cases. However, the mechanism behind treatment failure remains unclarified. We retrospectively examined the longitudinal profiles of immunoglobulin G (IgG) subclasses and/or the inhibitory activities of FVIII in plasma samples from 14 Japanese patients with congenital hemophilia A during hemostatic, FVIII replacement, and/or ITI therapies. In five patients, an increase in IgG4 was observed simultaneously with a decrease in IgG1 when the patient had a history of relatively high FVIII inhibitor titers, reflecting an apparent change in humoral immunity. In addition, we examined the reactivity and specificity of the patients' anti-FVIII IgG1 and IgG4 to FVIII domains by immunoblotting. Under our experimental conditions, plasma from three patients with historically higher inhibitor titers appeared to have high titers of antibodies against the A2-a2 domain, which did not necessarily correlate with ITI failure. These observations may improve scientific understanding of the immune response to infused FVIII in patients with hemophilia A.

Key Considerations for the Development of Safe and Effective SARS-CoV-2 Subunit Vaccine: A Peptide-Based Vaccine Alternative

COVID-19 is disastrous to global health and the economy. SARS-CoV-2 infection exhibits similar clinical symptoms and immunopathological sequelae to SARS-CoV infection. Therefore, much of the developmental progress on SARS-CoV vaccines can be utilized for the development of SARS-CoV-2 vaccines. Careful antigen selection during development is always of utmost importance for the production of effective vaccines that do not compromise recipient safety. This holds especially true for SARS-CoV vaccines, as several immunopathological disorders are associated with the activity of structural and nonstructural proteins encoded in the virus's genetic material. Whole viral protein and RNA-encoding full-length proteins contain both protective and "dangerous" sequences, unless pathological fragments are deleted. In light of recent advances, peptide vaccines may present a very safe and effective alternative. Peptide vaccines can avoid immunopathological pro-inflammatory sequences, focus immune responses on neutralizing immunogenic epitopes, avoid off-target antigen loss, combine antigens with different protective roles or mechanisms, even from different viral proteins, and avoid mutant escape by employing highly conserved cryptic epitopes. In this review, an attempt is made to exploit the similarities between SARS-CoV and SARS-CoV-2 in vaccine antigen screening, with particular attention to the pathological and immunogenic properties of SARS proteins.

Role of Regulatory Cells in Immune Tolerance Induction in Hemophilia A

The main complication of hemophilia A treatment is the development of neutralizing antibodies (inhibitors) against factor VIII (FVIII). Immune tolerance induction (ITI) is the prescribed treatment for inhibitor eradication, although its working mechanism remains unresolved. To clarify this mechanism, we compared blood samples of hemophilia A patients with and without inhibitors for presence of immunoregulatory cells and markers, including regulatory B-cells (Bregs), regulatory T-cells (Tregs), myeloid-derived suppressor cells (MDSCs), and expression of regulatory markers on T-cells (programmed cell death protein 1 [PD1], inducable T-cell costimulator, cytotoxic T-lymphocyte-associated protein 4 [CTLA4]), by use of flow cytometry. By cross-sectional analysis inhibitor patients (N = 20) were compared with inhibitor-negative (N = 28) and ex-inhibitor (N = 17) patients. In another longitudinal study, changes in immunoregulatory parameters were evaluated during ITI (N = 12) and compared with inhibitor-negative hemophilia A patients (N = 36). The frequency of Bregs, but not of Tregs nor MDSCs, was significantly reduced in inhibitor patients (3.2%) compared with inhibitor-negative (5.9%) and ex-inhibitor patients (8.9%; P < 0.01). CTLA4 expression on T-cells was also reduced (mean fluorescence intensity 133 in inhibitor versus 537 in inhibitor-negative patients; P < 0.01). Fittingly, in patients followed during ITI, inhibitor eradication associated with increased Bregs, increased Tregs, and increased expression of CTLA4 and PD1 on CD4+ T-cells. In conclusion, inhibitor patients express significantly lower frequency of Bregs and Tregs marker expression, which are restored by successful ITI. Our findings suggest that an existing anti-FVIII immune response is associated with deficits in peripheral tolerance mechanisms and that Bregs and changes in immunoregulatory properties of CD4+ T-cells likely contribute to ITI in hemophilia A patients with inhibitors.

Effect of the First Factor VIII Infusions on Immunological Biomarkers in Previously Untreated Patients with Hemophilia A from the HEMFIL Study

Hemophilia A (HA) is an inherited bleeding disorder which requires continuous replacement with factor (F) VIII concentrate. The main complication of HA is the development of neutralizing alloantibodies which inhibit FVIII activity (inhibitors). The objective of this study was to investigate the effect of the first FVIII infusions on immunological biomarkers in previously untreated patients with HA. Plasma samples were collected at enrollment before any FVIII infusion (T0) and at inhibitor development (INB +/T1) or up to 35 exposure days without inhibitors (INB -/T1). Anti-FVIII antibodies (immunoglobulin M, immunoglobulin G [IgG] 1, IgG3, and IgG4), chemokines (CCL2, CCL5, CXCL8, CXCL9, and CXCL10), and cytokines (interleukin [IL]-2, IL-4, IL-6, IL-10, interferon-γ, tumor necrosis factor, and IL-17) were assessed. A total of 71 children with severe HA were included, of whom 28 (39.4%) developed inhibitors. Plasma levels of anti-FVIII IgG4, IL-6, and CXCL8 were higher at INB +/T1 when compared with INB -/T1. This group presented a mixed cytokine profile and higher plasma levels of CXCL9 and CXL10 when compared with INB +/T1. We conclude that exposure to FVIII triggers a proinflammatory response mediated by IL-6 and CXCL8 in patients with HA who developed inhibitors. Regardless of inhibitor status, the immune system of all HA patients is stimulated after infusions of FVIII.

Validation of a Multiplex Assay for Measuring Specific IgG4

PURPOSE

Specific IgG4 (sIgG4) increases with allergen specific immunotherapy and may reflect a state of immune tolerance in food allergy. While ImmunoCAP® has been widely used to measure sIgG4 to a single allergen, PROTIA™ Specific IgG4® has been designed as a multiplex assay for measuring sIgG4. This study sought to validate this assay in comparison to ImmunoCAP®.

MATERIALS AND METHODS

Measurements of sIgG4 were compared between PROTIA™ Specific IgG4® and ImmunoCAP® using sera from 519 allergy patients (asthma: 114, allergic rhinitis: 318, food allergy: 146) with 731 paired tests. sIgG4 was measured against nine inhalant allergens (Dermatophagoides pteronyssinus, Dermatophagoides farinae, cat dander, dog dander, birch pollen, oak pollen, ragweed pollen, mugwort pollen, and Alternaria alternata spores) and nine food allergens (egg white, casein, wheat, peanut, walnut, crab, shrimp, apple, and peach).

RESULTS

PROTIA™ Specific IgG4® showed 95.6% agreement rate with ImmunoCAP® in the positivity comparison. For sIgG4 positivity to each individual allergen, an agreement rate of more than 84.8% was observed. In Cohen's kappa analysis, these assays displayed substantial correlations [Cohen's kappa coefficient (κ) ≥0.699], except for shrimp (κ=0.448). Furthermore, both assays displayed strong correlations in quantitative comparisons [correlation coefficients value (ρ) ≥0.8014], except for apple (ρ=0.6571, p=0.175). Serial dilution tests also showed consistency between the assays.

CONCLUSION

PROTIA™ Specific IgG4® showed high consistency with ImmunoCAP® in measuring sIgG4. This assay is applicable to various clinical fields, including allergen immunotherapy and food allergy.

Prevalence and Incidence of Non-neutralizing Antibodies in Congenital Hemophilia A- A Systematic Review and Meta-Analysis

Objectives: In hemophilia A the presence of non-neutralizing antibodies (NNAs) against Factor VIII (FVIII) may predict the development of neutralizing antibodies (inhibitors) and accelerate the clearance of administrated FVIII concentrates. This systematic review aimed to assess: (1) the prevalence and incidence of NNAs in patients with congenital hemophilia without inhibitors and (2) the association between NNAs and patient and treatment characteristics.

Methods: We conducted a search in MEDLINE, Embase, Web of Science and the Cochrane database. We included cross-sectional and longitudinal studies reporting on NNAs in patients with hemophilia A and B, who were inhibitor-negative at the start of the observation period. Data were extracted on: hemophilia type and severity, patient and treatment characteristics, NNA prevalence and incidence, NNA assays and inhibitor development. Two independent reviewers performed study selection, data extraction and risk of bias assessment, using adapted criteria of the Joanna Briggs Institute. Studies were classified as high-quality when ≥5/9 criteria were met. NNA assays were classified as high-quality when both quality criteria were met: (1) use of positive controls and (2) competition with FVIII to establish FVIII-specificity. We reported NNA prevalence and incidence for each study. The pooled NNA prevalence was assessed for well-designed studies in previously treated patients, employing high-quality NNA assays.

Results: We included data from 2,723 inhibitor-negative patients with hemophilia A, derived from 28 studies. Most studies were cross-sectional (19/28) and none reported on NNAs in hemophilia B. Study design was of high quality in 16/28 studies and the NNA assay quality was high in 9/28 studies. Various NNA assays were used, predominantly ELISA (18/28) with different cut-off values. We found a large variety in NNA prevalence (Range, 0–100%). The pooled NNA prevalence in high-quality studies was 25% (95% CI, 16–38%). The incidence of new NNA development was reported in one study (0.01 NNA per person-exposure day).

Conclusion: This systematic review identified studies that were heterogeneous in study design, patient population and NNA assay type, with NNA prevalence ranging from 0 to 100% in inhibitor-negative patients with hemophilia A. The pooled NNA prevalence was 25% in high-quality studies including only previously treated patients and performing high-quality NNA assays.

Inhibitors: A Need for Eradication?

The development of inhibitors against factor VIII (FVIII) concentrates represents a significant treatment complication for hemophilia. Immune tolerance induction (ITI) therapy eradicates inhibitors in 60-80% of patients, resulting in a normal FVIII response. This article, based on presentations at the 6th International Coagulation Meeting, held in Barcelona, Spain, in September 2017, provides an overview of management approaches for patients with inhibitors and briefly tabulates four cases of ITI therapy (first-line or rescue ITI therapy in pediatric and adult patients) with successful outcomes. Switching FVIII product from recombinant FVIII to plasma-derived FVIII/VWF concentrate may be helpful in eradicating inhibitors. The rate of decline of inhibitor titer in the initial stages of ITI therapy is a good indicator of the success or failure of therapy, although prognostic biomarkers are needed. The development of the bispecific monoclonal antibody emicizumab, which was recently shown to reduce bleeding in inhibitor patients, offers a potential alternative therapeutic option. However, the benefits of inhibitor eradication, including a wider choice of cheaper therapeutic products for preventing and treating bleeds, suggest that at least one attempt of ITI therapy should be offered to patients who develop inhibitors.

Antibodies against recombinant alpha-galactosidase A in Fabry disease: Subclass analysis and impact on response to treatment

BACKGROUND

Treatment of Fabry disease (FD) with recombinant alpha-galactosidase A (r-αGAL A) is complicated by the formation of anti-drug antibodies in the majority of male patients with the classical disease phenotype. Detailed information regarding antibody subtypes, onset and persistence of antibody development and their effect on treatment efficacy is sparse.

METHODS

A retrospective study was carried out in 39 male patients with classical FD, treated with either agalsidase-alfa or agalsidase-beta (mean follow up of 10 years). With six to twelve months intervals plasma-induced in vitro inhibition of enzyme activity, lysoglobotriaosylsphingosine (lysoGb3) levels and renal function were assessed. In a subset of 12 patients, additionally anti- r-αGAL A IgM, IgA and IgG1, 2, 3 and 4 levels were analyzed.

RESULTS

In 23 out of 39 patients, plasma-induced in vitro inhibition of r-αGAL A activity was observed (inhibition-positive). The inhibition titer was strongly negatively correlated to the decrease in lysoGb3: agalsidase-alfa (FE_{log10(inhibition)} = -10.3, P ≤.001), agalsidase-beta (FE_{log10(inhibition)} = -4.7, P ≤.001). Inhibition-positive patients had an accelerated decline in renal function (FE = 1.21, p = .042). During treatment IgG1 anti-r-αGAL A levels increased only in inhibition-positive patients (p = .0045). IgG4 anti-r-αGAL A antibodies developed in 7 out of 9 inhibition-positive patients. Other antibody subclasses were either not present or too low to quantify.

CONCLUSION

Development of inhibiting antibodies against r-αGAL A negatively affects the biochemical response to ERT and resulted in an accelerated decline in renal function. The presence of IgG1 and IgG4 anti-r-αGAL A antibodies is associated with in vitro αGAL A activity inhibition.

Factor VIII with a 237 amino acid B-domain has an extended half-life in F8-knockout mice

Essentials Factor (F)VIII with an intermediate-length B-domain showed higher levels in murine gene therapy. FVIII with different B-domain lengths were analysed. FVIII variants with B-domains between 186 and 240 amino acids (aa) have extended half-life in mice. Reduced cell binding of FVIII with a 237aa B-domain may explain the extended half-life. SUMMARY: Background Factor VIII consists of the A1-domain, A2-domain, B-domain, A3-domain, C1-domain, and C2-domain. FVIII with an intermediate-length B-domain of 226 amino acids (aa) has previously been evaluated in murine gene therapy studies. Objective To characterize FVIII with intermediate-length B-domains in vitro and in vivo in F8-knockout (KO) mice. Methods and results FVIII molecules with B-domains of 186-240aa had longer half-lives in F8-KO mice than FVIII molecules with shorter or longer B-domains. FVIII with a B-domain containing the 225 N-terminal aa fused to the 12 C-terminal aa of the wild-type B-domain (FVIII-237) had a 1.6-fold extended half-life in F8-KO mice as compared with FVIII with a 21aa B-domain (FVIII-21). The in vitro and in vivo activity of FVIII-237 were comparable to those of FVIII-21, as was binding to von Willebrand factor. Cell binding to LDL receptor-related protein 1 (LRP-1)-expressing cells was markedly reduced for FVIII-237 as compared with FVIII-21, whereas the affinity for LRP-1 was not reduced in surface plasmon resonance (SPR) studies. FVIII-21 cell binding and internalization could be inhibited by a fragment consisting of the 226 N-terminal aa of the FVIII B-domain, and SPR analysis suggested that this B-domain fragment might bind with weak affinity to FVIII-21. Conclusion Reduced cell binding of FVIII-237 might explain the observed extended half-life in F8-KO mice. This may contribute to the increased FVIII levels measured in murine gene therapy studies using FVIII constructs with similar B-domain lengths.

Heat-Killed Saccharomyces cerevisiae, A Dectin-1 Agonist, Selectively Induces IgG4 Production by Human B Cells

Dectin-1 is a major receptor that recognizes fungal cell wall β-glucan. We previously reported that heat-killed Saccharomyces cerevisiae (HKSC), a Dectin-1 agonist, selectively induces IgG1 class switching in mouse B cells. Dectin-1 is also expressed on human B cells; however, Dectin-1 function in human B cells remains unknown. This study aimed to investigate the direct effect of in vitro stimulation using HKSC on Ig class switching in human B cells. HKSC selectively induced the expression of germline γ4 transcripts (GLTγ4) by human B cell line 2E2, and HKSC significantly augmented GLTγ4 promoter activity. Moreover, HKSC selectively enhanced GLTγ4 expression and IgG4 production by anti-CD40-activated human tonsillar resting B cells. Thus, these results suggest that Dectin-1 maybe involved in selective IgG4 class switching by human B cells.

New insights into immune cells cross-talk during IgG4-related disease

Immunoglobulin G4-related disease (IgG4-RD) is a newly acknowledged entity, characterized by an immune-mediated fibro-inflammatory process affecting virtually all organs, with infiltration of IgG4⁺ bearing plasma cells. Until today the pathogenesis of IgG4-RD remains unknown. Treatment with anti-CD20 monoclonal antibodies efficiently induced remission and attenuated the secretory phenotype of myofibroblasts responsible of uncontrolled collagen deposition. This supports the pathogenic role of the adaptive immunity, particularly B cell compartment and B cell/T cell interaction. Latest studies have also highlighted the importance of innate immune system that has been underestimated before and the key role of a specific T cell subset, T follicular helper cells that are involved in IgG4-class-switching and plasmablast differentiation. In this review, we aim to review the most recent knowledge of innate immunity, T and B cells involvement in IgG4-RD, and introduce tertiary lymphoid organs (TLO) as a potential marker of relapse in this condition.

IgG4 anti-infliximab in treated patients: Clinical impact and temporal evolution

BACKGROUND

Infliximab (IFX) carries potential risk of immunogenicity with the production of anti-drug antibodies (ADA). ADA may belong to different isotypes and are usually measured by ELISA bridging assay. This test is not designed to detect IgG4 antibodies. The aim was to measure IgG4 anti-IFX antibodies in a cohort of IFX-treated patients and to evaluate their relationship with ADA and their clinical impact.

METHODS

Anti-drug antibodies were detected using a bridging ELISA in the serum of 222 treated patients with different clinical outcomes to IFX. The same samples were analyzed for IgG4 anti-IFX antibodies using an experimental ImmunoCAP assay with reduced serum IgG4 background levels. A longitudinal evaluation was performed in a subgroup of 38 patients to define the temporal evolution of IgG4 anti-IFX.

RESULTS

IgG4 anti-IFX was found in 26.6% of patients. Eighty of 222 patients were ADA+ (36%) and the majority (57/80, 71.3%) had IgG4 anti-IFX. Two IgG4-positive but ADA-negative patients were identified. IgG4 anti-IFX levels correlated with the serum levels of ADA. IgG4 anti-IFX was more common in both reactive and nonresponder patients than in tolerant/responder patients. Patients who had experienced IgE-mediated reactions displayed significantly higher IgG4 anti-IFX than IgE-negative reactive patients. The majority of patients tested positive for IgG4 anti-IFX after the first seven infusions.

CONCLUSIONS

IgG4 anti-IFX is common in treated patients and a large part of ADA producing patients produce IgG4 antibodies. The IgG4 anti-IFX response does not prevent hypersensitivity reactions to IFX and correlates with the IgE anti-IFX response.

Distinguishing lupus anticoagulants from factor VIII inhibitors in haemophilic and non-haemophilic patients

INTRODUCTION

Accurate diagnosis of an inhibitor, a neutralizing antibody to infused factor VIII (FVIII), is essential for appropriate management of haemophilia A (HA). Low-titre inhibitors may be difficult to diagnose due to high rates of false-positive inhibitor results in that range. Transient low-titre inhibitors and false-positive inhibitors may be due to the presence of a lupus anticoagulant (LA) or other non-specific antibodies. Fluorescence immunoassay (FLI) to detect antibodies to FVIII is a sensitive method to identify inhibitors in HA. Evaluations of antibody profiles by various groups have demonstrated that haemophilic inhibitors detected by Nijmegen-Bethesda (NBA) and chromogenic Bethesda (CBA) assays correlate with positivity for anti-FVIII immunoglobulin (Ig) G1 and G4.

AIM

This study sought to determine whether FLI could distinguish false-positive FVIII inhibitor results related to LAs from clinically relevant FVIII inhibitors in HA patients.

METHODS

Samples from haemophilic and non-haemophilic subjects were tested for LA, specific FVIII inhibitors by NBA and CBA, and anti-FVIII immunoglobulin profiles by FLI.

RESULTS

No samples from LA-positive non-haemophilic subjects were positive by FLI for anti-FVIII IgG4. Conversely, 91% of NBA-positive samples from haemophilia subjects were positive for anti-FVIII IgG4. Two of 11 haemophilia subjects had samples negative for anti-FVIII IgG4 and CBA, which likely represented LA rather than FVIII inhibitor presence.

CONCLUSIONS

Assessment of anti-FVIII profiles along with the CBA may be useful to distinguish a clinically relevant low-titre FVIII inhibitor from a transient LA in HA patients.

Anti-Drug Antibodies: Emerging Approaches to Predict, Reduce or Reverse Biotherapeutic Immunogenicity

The development of anti-drug antibodies (ADAs) following administration of biotherapeutics to patients is a vexing problem that is attracting increasing attention from pharmaceutical and biotechnology companies. This serious clinical problem is also spawning creative research into novel approaches to predict, avoid, and in some cases even reverse such deleterious immune responses. CD4⁺ T cells are essential players in the development of most ADAs, while memory B-cell and long-lived plasma cells amplify and maintain these responses. This review summarizes methods to predict and experimentally identify T-cell and B-cell epitopes in therapeutic proteins, with a particular focus on blood coagulation factor VIII (FVIII), whose immunogenicity is clinically significant and is the subject of intensive current research. Methods to phenotype ADA responses in humans are described, including T-cell stimulation assays, and both established and novel approaches to determine the titers, epitopes and isotypes of the ADAs themselves. Although rational protein engineering can reduce the immunogenicity of many biotherapeutics, complementary, novel approaches to induce specific tolerance, especially during initial exposures, are expected to play significant roles in future efforts to reduce or reverse these unwanted immune responses.

Laboratory testing for factor VIII and IX inhibitors in haemophilia: A review

Inhibitors are antibodies directed against haemophilia treatment products which interfere with their function. Factor VIII (FVIII) inhibitors in haemophilia A and factor IX (FIX) inhibitors in haemophilia B are significant clinically when they require a change in a patient's treatment regimen. Their persistence may increase morbidity and mortality. Multiple laboratory tests are now available for detecting and understanding inhibitors in haemophilia. Inhibitors are traditionally measured by their interference in clotting or chromogenic factor assays. They may also be detected using immunologic assays, such as enzyme-linked immunosorbent assay or fluorescence immunoassay. Anti-FVIII or anti-FIX antibodies of IgG4 subclass best correlate with the presence of functional inhibitors. Improvements in inhibitor measurement have been recently introduced. Preanalytical heat treatment of patient specimens allows testing of patients without delaying treatment. Use of chromogenic and immunologic assays may aid in identification of false-positive results, which are frequent among low-titre inhibitors. Validated reagent substitutions can be used to reduce assay cost. New methods for defining assay positivity and reporting low-titre inhibitors have been suggested. Challenges remain in the areas of quality control, assay standardization, monitoring of patients undergoing immune tolerance induction therapy and testing in the presence of modified and novel treatment products.

Coagulation factor VII variants resistant to inhibitory antibodies

Replacement therapy is currently used to prevent and treat bleeding episodes in coagulation factor deficiencies. However, structural differences between the endogenous and therapeutic proteins might increase the risk for immune complications. This study was aimed at identifying factor (F)VII variants resistant to inhibitory antibodies developed after treatment with recombinant activated factor VII (rFVIIa) in a FVII-deficient patient homozygous for the p.A354V-p.P464Hfs mutation, which predicts trace levels of an elongated FVII variant in plasma. We performed fluorescent bead-based binding, ELISA-based competition as well as fluorogenic functional (activated FX and thrombin generation) assays in plasma and with recombinant proteins. We found that antibodies displayed higher affinity for the active than for the zymogen FVII (half-maximal binding at 0.54 ± 0.04 and 0.78 ± 0.07 BU/ml, respectively), and inhibited the coagulation initiation phase with a second-order kinetics. Isotypic analysis showed a polyclonal response with a large predominance of IgG1. We hypothesised that structural differences in the carboxyl-terminus between the inherited FVII and the therapeutic molecules contributed to the immune response. Intriguingly, a naturally-occurring, poorly secreted and 5-residue truncated FVII (FVII-462X) escaped inhibition. Among a series of truncated rFVII molecules, we identified a well-secreted and catalytically competent variant (rFVII-464X) with reduced binding to antibodies (half-maximal binding at 0.198 ± 0.003 BU/ml) as compared to the rFVII-wt (0.032 ± 0.002 BU/ml), which led to a 40-time reduced inhibition in activated FX generation assays. Taken together our results provide a paradigmatic example of mutation-related inhibitory antibodies, strongly support the FVII carboxyl-terminus as their main target and identify inhibitor-resistant FVII variants.

Limit of detection and threshold for positivity of the Centers for Disease Control and Prevention assay for factor VIII inhibitors

Essentials Immunologic methods detect factor VIII (FVIII) antibodies in some inhibitor-negative specimens. Specimens were tested by modified Nijmegen-Bethesda assay (NBA) and fluorescence immunoassay. The NBA with preanalytical heat inactivation detects FVIII inhibitors down to 0.2 NBU. IgG4 frequency validates the established threshold for positivity of ≥ 0.5 NBU for this NBA.

SUMMARY

Background The Bethesda assay for measurement of factor VIII inhibitors called for quantification of positive inhibitors by using dilutions producing 25-75% residual activity (RA), corresponding to 0.4-2.0 Bethesda units, with the use of 'more sensitive methods' for samples with RA closer to 100% being recommended. The Nijmegen modification (Nijmegen-Bethesda assay [NBA]) changed the reagents used but not these calculations. Some specimens negative by the NBA have been shown to have FVIII antibodies detectable with sensitive immunologic methods. Objective To examine the performance at very low inhibitor titers of the Centers for Disease Control and Prevention (CDC)-modified NBA (CDC-NBA), which includes preanalytic heat inactivation to liberate bound anti-FVIII antibodies. Methods Specimens with known inhibitors were tested with the CDC-NBA. IgG4 anti-FVIII antibodies were measured by fluorescence immunoassay (FLI). Results Diluted inhibitors showed linearity below 0.4 Nijmegen-Bethesda units (NBU). With four statistical methods, the limit of detection of the CDC-NBA was determined to be 0.2 NBU. IgG4 anti-FVIII antibodies, which correlate most strongly with functional inhibitors, were present at rates above the background rate of healthy controls in specimens with titers ≥ 0.2 NBU and showed an increase in frequency from 14.3% at 0.4 NBU to 67% at the established threshold for positivity of 0.5 NBU. Conclusions The CDC-NBA can detect inhibitors down to 0.2 NBU. The FLI, which is more sensitive, demonstrates anti-FVIII IgG4 in some patients with negative (< 0.5) NBU. The sharp increase in IgG4 frequency between 0.4 and 0.5 NBU validates the established threshold for positivity of ≥ 0.5 NBU for the CDC-NBA, supporting the need for method-specific thresholds.

First report of real-time monitoring of coagulation function potential and IgG subtype of anti-FVIII autoantibodies in a child with acquired hemophilia A associated with streptococcal infection and amoxicillin

We describe an 8-year-old boy with acquired hemophilia A (AHA) associated with streptococcal infection and amoxicillin. Laboratory data revealed low factor VIII activity (FVIII:C, 1.5 IU/dl), and FVIII inhibitor (15.9 BU/ml). Comprehensive coagulation function assays, including rotation thromboelastometry (ROTEM^®), revealed a markedly prolonged clotting time. Thrombin and plasmin generation (TG/PG) appeared to be moderately impaired. The inhibitor epitope of his anti-FVIII autoantibody recognized light and heavy chains. He was treated with Novoseven^® and prednisolone, resulting in rapid improvement. ROTEM showed the return of coagulation time to normal level on day 20, and TG gradually improved. PG was moderately reduced in the clinical early phase, but improved at day 20. The patient's IgG subtype was IgG₄ at onset. IgG₁ was transiently positive on day 20, but negative on day 46. FVIII inhibitor gradually decreased and was completely absent after day 46, along with the elevated FVIII:C. IgG4 was again elevated on day 83, followed by a rapid decrease, indicative of the presence of non-neutralizing antibody, which remains currently undetected. We for the first time report changes in comprehensive coagulation function and IgG subtype of anti-FVIII antibody in a rare pediatric case of AHA.

CD32 inhibition and high dose of rhFVIII suppress murine FVIII-specific recall response by distinct mechanisms in vitro

Development of neutralising antibodies (inhibitors) against factor VIII (FVIII) is a frequent and severe complication of replacement therapy in haemophilia A. Previous data from haemophilia A mouse model demonstrates that both CD32 inhibition and high doses of rhFVIII prevent the differentiation of FVIII-specific memory B cells (MBCs) into antibody secreting cells (ASCs). Here, cellular targets responsible for the suppression of ASC formation by means of CD32 inhibition and high dose of rhFVIII were analysed. We investigated apoptosis on FVIII-specific MBCs using a pan caspases inhibitor, and screened for defects in rhFVIII presentation by analysing T cell release of Th1- and Th2-cytokines in vitro. Although high dose of rhFVIII suppressed ASC formation, cytokine response was not affected. Upon re-stimulation of splenocytes with high dose of rhFVIII, prevention of apoptosis fully restored the FVIII-specific recall response. In contrast, genetic deletion or inhibition of CD32 significantly altered Th1- and Th2-response. CD32 blockade and inhibition of apoptosis resulted in a partial rescue of FVIII-specific ASCs. Normal cytokine secretion could not be restored. In conclusion, suppression of FVIII-specific recall response by CD32 and high doses of rhFVIII is mediated by distinct mechanisms. High dose of rhFVIII induces apoptosis in FVIII-specific MBCs but does not influence FVIII-specific T cell response. CD32 blockade, however, may suppress the FVIII-specific recall response by two ways: i) increasing apoptosis of FVIII-specific MBCs and ii) disturbing FVIII-specific T cell response by modulating presentation of rhFVIII to CD4⁺ T cells in vitro.

Nonneutralizing antibodies against factor VIII and risk of inhibitor development in severe hemophilia A

Nonneutralizing antibodies against FVIII are detected in untreated or minimally treated patients with hemophilia A. The presence of nonneutralizing antibodies is associated with a substantially increased risk of inhibitor development.

Enhanced uptake of blood coagulation factor VIII containing immune complexes by antigen presenting cells

Essentials Anti-factor (F) VIII antibody formation is a major complication in the treatment of hemophilia A. We investigated uptake of FVIII and FVIII immune complex by bone marrow derived dendritic cells. Immune complex formation increased uptake of FVIII 3-4 fold in a Fcγ receptor dependent manner. FVIII immune complex binding to Fcγ receptors may modulate immune tolerance induction.

SUMMARY

Background A major complication in the treatment of hemophilia A is the development of inhibitory antibodies targeting coagulation factor VIII (FVIII). Eradication of these inhibitors can be established by immune tolerance induction (ITI), which consists of daily administration of high dosages of FVIII. FVIII immune complexes (FVIII-IC) could be formed following FVIII infusion in patients with pre-existing anti-FVIII antibodies. Objectives Here we studied endocytosis of FVIII-IC by bone marrow-derived dendritic cells (BMDCs). Methods BMDCs were pulsed with FVIII/FVIII-IC and uptake was assessed by flow cytometry and confocal imaging. Results BMDCs were able to efficiently internalize FVIII-IC in a dose-dependent manner, 3-4-fold more efficiently when compared with equimolar concentrations of non-complexed FVIII. Uptake of FVIII-IC, but not FVIII alone, could be inhibited with anti-Fcγ receptor (FcγR) antibody 2.4G2, indicating functional involvement of FcγR. No internalization of FVIII-IC was observed in BMDCs lacking FcγRI, FcγRIIb, FcγRIII and FcγRIV. Genetic ablation of FcγRIIb, FcγRIII or FcγRIV individually did not affect the ability of anti-FVIII IgG to promote the uptake of FVIII. BMDCs lacking FcγRI showed lower FVIII-IC uptake levels when compared with other single FcγR null BMDCs. Expression of the inhibitory FcγRIIb alone was sufficient to internalize FVIII-IC more efficiently than FVIII. Conclusions FcγR are critical in the internalization of FVIII-IC by BMDCs and multiple FcγR can contribute independently to this process. Our findings provide a basis for future studies to address whether the outcome of ITI is dependent on the interplay between FVIII-IC and inhibitory and activating FcγR.

Achievements, challenges and unmet needs for haemophilia patients with inhibitors: Report from a symposium in Paris, France on 20 November 2014

Over the past 20 years, there have been many advances in haemophilia treatment that have allowed patients to take greater control of their disease. However, the development of factor VIII (FVIII) inhibitors is the greatest complication of the disease and a challenge in the treatment of haemophilia making management of bleeding episodes difficult and surgical procedures very challenging. A meeting to discuss the unmet needs of haemophilia patients with inhibitors was held in Paris on 20 November 2014. Topics discussed were genetic and non-genetic risk factors for the development of inhibitors, immunological aspects of inhibitor development, FVIII products and inhibitor development, generation and functional properties of engineered antigen-specific T regulatory cells, suppression of immune responses to FVIII, prophylaxis in haemophilia patients with inhibitors, epitope mapping of FVIII inhibitors, current controversies in immune tolerance induction therapy, surgery in haemophilia patients with inhibitors and future perspectives for the treatment of haemophilia patients with inhibitors. A summary of the key points discussed is presented in this paper.

FVIII-binding IgG modulates FVIII half-life in patients with severe and moderate hemophilia A without inhibitors

FVIII-specific IgG antibodies modulate FVIII half-life in patients with hemophilia A, independent of VWF antigen levels and age. Screening for FVIII-specific IgG may be beneficial in tailoring FVIII prophylactic regimens for hemophilia A patients.

Antibody response to recombinant human coagulation factor VIII in a new rat model of severe hemophilia A

BACKGROUND

Neutralizing antibodies toward FVIII replacement therapy (inhibitors) are the most serious treatment-related complication in hemophilia A (HA). A rat model of severe HA (F8(-/-) ) has recently been developed, but an immunological characterization is needed to determine the value of using the model for research into inhibitor development.

OBJECTIVES

Characterize the antibody response towards recombinant human coagulation factor VIII (rhFVIII) in the HA rat, following a human prophylactic dosing regimen.

METHODS

Two identical studies were performed, which included a total of 17 homozygous HA rats (F8(-/-) , 0% FVIII activity), 12 heterozygous rats (F8(+/-) ), and 12 wild-type (F8(+/+) ) rats. All rats received intravenous injections of rhFVIII at 50 IU kg(-1) twice weekly for 4 weeks. Predosing blood samples were analyzed for binding and neutralizing anti-rhFVIII antibodies at weeks 1-7.

RESULTS

In both studies, antibodies developed after 4-6 administrations of rhFVIII, and neutralizing antibodies reached levels similar to human patients (range 1-111 BU, median 6.0 BU) at the end of the study. There was no significant difference between the two studies or between genotypes in time to response or levels reached for binding and neutralizing antibodies. Interestingly, early spontaneous bleeds were associated with a faster antibody response.

CONCLUSIONS

Following intravenous administration of human FVIII, according to a clinical prophylaxis regimen, a robust and reproducible antibody response is seen in this HA rat model, suggesting that the model is useful for intervention studies with the aim of suppressing, delaying, or preventing the inhibitor response. Also, bleeds seem to have an adjuvant effect on the immune response.

Strategies to target long-lived plasma cells for treating hemophilia A inhibitors

Long-lived plasma cells (LLPCs) can persistently produce anti-factor VIII (FVIII) antibodies which disrupt therapeutic effect of FVIII in hemophilia A patients with inhibitors. The migration of plasma cells to BM where they become LLPCs is largely controlled by an interaction between the chemokine ligand CXCL12 and its receptor CXCR4. AMD3100 combined with G-CSF inhibit their interactions, thus facilitating the mobilization of CD34(+) cells and blocking the homing of LLPCs. These reagents were combined with anti-CD20 to reduce B-cells and the specific IL-2/IL-2mAb (JES6-1) complexes to induce Treg expansion for targeting anti-FVIII immune responses. Groups of mice primed with FVIII plasmid and protein respectively were treated with the combined regimen for six weeks, and a significant reduction of anti-FVIII inhibitor titers was observed, associated with the dramatic decrease of circulating and bone marrow CXCR4(+) plasma cells. The combination regimens are highly promising in modulating pre-existing anti-FVIII antibodies in FVIII primed subjects.

von Willebrand factor binds to the surface of dendritic cells and modulates peptide presentation of factor VIII

It has been proposed that von Willebrand factor might affect factor VIII immunogenicity by reducing factor VIII uptake by antigen presenting cells. Here we investigate the interaction of recombinant von Willebrand factor with immature monocyte-derived dendritic cells using flow cytometry and confocal microscopy. Surprisingly, von Willebrand factor was not internalized by immature dendritic cells, but remained bound to the cell surface. As von Willebrand factor reduces the uptake of factor VIII, we investigated the repertoire of factor VIII presented peptides when in complex with von Willebrand factor. Interestingly, factor VIII-derived peptides were still abundantly presented on major histocompatibility complex class II molecules, even though a reduction of factor VIII uptake by immature dendritic cells was observed. Inspection of peptide profiles from 5 different donors showed that different core factor VIII peptide sequences were presented upon incubation with factor VIII/von Willebrand factor complex when compared to factor VIII alone. No von Willebrand factor peptides were detected when immature dendritic cells were pulsed with different concentrations of von Willebrand factor, confirming lack of von Willebrand factor endocytosis. Several von Willebrand factor derived peptides were recovered when cells were pulsed with von Willebrand factor/factor VIII complex, suggesting that factor VIII promotes endocytosis of small amounts of von Willebrand factor by immature dendritic cells. Taken together, our results establish that von Willebrand factor is poorly internalized by immature dendritic cells. We also show that von Willebrand factor modulates the internalization and presentation of factor VIII-derived peptides on major histocompatibility complex class II.

Rapid acquisition of immunologic tolerance to factor VIII and disappearance of anti-factor VIII IgG4 after prophylactic therapy in a hemophilia A patient with high-titer factor VIII inhibitor

We report an 11-month-old boy with severe hemophilia A who had regular exposure to factor VIII (FVIII) intended to reduce the risk of developing an inhibitor. He developed a high-titer inhibitor (peak titer 19 BU) that disappeared within 6 weeks of starting immune tolerance induction (ITI). Anti-FVIII IgG4 peaked briefly compared with anti-FVIII IgG1 and the Bethesda titer. Neither rapid resolution of an inhibitor after prophylaxis nor this behavior of anti-FVIII IgG4 has been previously reported. Transient anti-FVIII IgG4 may be a marker of an attenuated anti-FVIII response induced by prophylactic FVIII therapy.

Factor VIII Interacts with the Endocytic Receptor Low-density Lipoprotein Receptor-related Protein 1 via an Extended Surface Comprising "Hot-Spot" Lysine Residues

Lysine residues are implicated in driving the ligand binding to the LDL receptor family. However, it has remained unclear how specificity is regulated. Using coagulation factor VIII as a model ligand, we now study the contribution of individual lysine residues in the interaction with the largest member of the LDL receptor family, low-density lipoprotein receptor-related protein (LRP1). Using hydrogen-deuterium exchange mass spectrometry (HDX-MS) and SPR interaction analysis on a library of lysine replacement variants as two independent approaches, we demonstrate that the interaction between factor VIII (FVIII) and LRP1 occurs over an extended surface containing multiple lysine residues. None of the individual lysine residues account completely for LRP1 binding, suggesting an additive binding model. Together with structural docking studies, our data suggest that FVIII interacts with LRP1 via an extended surface of multiple lysine residues that starts at the bottom of the C1 domain and winds around the FVIII molecule.

Anti-A2 and anti-A1 domain antibodies are potential predictors of immune tolerance induction outcome in children with hemophilia A

BACKGROUND

Hemophilia A (HA) is a congenital bleeding disorder resulting from factor VIII deficiency. The most serious complication of HA management is the appearance of inhibitory antibodies (Abs) against injected FVIII concentrates. To eradicate inhibitors, immune tolerance induction (ITI) is usually attempted, but it fails in up to 30% of cases. Currently, no undisputed predictive marker of ITI outcome is available to facilitate the clinical decision.

OBJECTIVES

To identify predictive markers of ITI efficacy.

METHODS

The isotypic and epitopic repertoires of inhibitory Abs were analyzed in plasma samples collected before ITI initiation from 15 children with severe HA and high-titer inhibitors, and their levels were compared in the two outcome groups (ITI success [n = 7] and ITI failure [n = 8]). The predictive value of these candidate biomarkers and of the currently used indicators (inhibitor titer and age at ITI initiation, highest inhibitor titer before ITI, and interval between inhibitor diagnosis and ITI initiation) was then compared by statistical analysis (Wilcoxon test and receiver receiver operating characteristic [ROC] curve analysis).

RESULTS

Whereas current indicators seemed to fail in discriminating patients in the two outcome groups (ITI success or failure), anti-A1 and anti-A2 Ab levels before ITI initiation appeared to be good potential predictive markers of ITI outcome (P < 0.018). ROC analysis showed that anti-A1 and anti-A2 Abs were the best at discriminating between outcome groups (area under the ROC curve of > 0.875).

CONCLUSION

Anti-A1 and anti-A2 Abs could represent new promising tools for the development of ITI outcome prediction tests for children with severe HA.

A longitudinal evaluation of anti-FVIII antibodies demonstrated IgG4 subclass is mainly correlated with high-titre inhibitor in haemophilia A patients

The development of inhibitory antibodies against factor VIII (FVIII) (inhibitor) is the major complication in haemophilia A patients. The FVIII-binding antibodies development comprises a polyclonal immunoglobulin (Ig) G response. Recent studies showed strong correlation between the presence of neutralizing anti-FVIII antibodies (inhibitors) and IgG4 subclass. The aim of this study was to evaluate anti-FVIII IgG subclasses in haemophilia A patients with inhibitor both in a cross-sectional and in a longitudinal analysis. Inhibitors were determined by Nijmegen-Bethesda assay. Anti-FVIII IgG subclasses were performed by ELISA, and samples from 20 healthy individuals were used to validate the test. We studied 25 haemophilia A patients with inhibitor, previously treated exclusively with plasma-derived FVIII concentrates or bypassing agents. The IgG subclasses distributions were evaluated in two groups of patients classified according to inhibitor response. IgG1 and IgG4 antibodies were most prominent in haemophilia A patients with inhibitors when compared with IgG2 and IgG3. This study reports for the first time the behaviour of FVIII-binding IgG1 and IgG4 subclasses in a longitudinal analysis, in a clinical setting, of high-response inhibitor haemophilia A patients, showing the correlation of IgG4 and the inhibitor titres. In spite of being considered a non-pathologic antibody subclass with anti-inflammatory properties in other situations, IgG4 is correlated with the presence of high-titre inhibitor in the haemophilia setting. The comprehension of the IgG4 role in immune response may be crucial to establish the process for designing specific tolerance to FVIII.

New predictive approaches for ITI treatment

Immune tolerance induction (ITI) therapy in patients with haemophilia A and inhibitors constitutes a huge burden for affected patients and families and poses a large economic burden for a chronic disease. Concerted research efforts are attempting to optimize the therapeutic approach to the prevention and eradication of inhibitors. The Italian ITI Registry has provided data on 110 patients who completed ITI therapy as at July 2013. Analysis of independent predictors of success showed that, together with previously recognized factors - namely inhibitor titre prior to ITI, historical peak titre and peak titre on ITI - the type of causative FVIII gene mutation also contributes to the identification of patients with good prognosis and may be useful to optimize candidate selection and treatment regimens. Numerous studies have demonstrated that inhibitor reactivity against different FVIII products varies and is lower against concentrates containing von Willebrand factor (VWF). An Italian study compared inhibitor titres against a panel of FVIII concentrates in vitro and correlated titres with the capacity to inhibit maximum thrombin generation as measured by the thrombin generation assay (TGA). Observations led to the design of the PredictTGA study which aims to correlate TGA results with epitope specificity, inhibitor reactivity against different FVIII concentrates and clinical data in inhibitor patients receiving FVIII in the context of ITI or as prophylactic/on demand treatment. At the immunological level, it is known that T cells drive inhibitor development and that B cells secrete FVIII-specific antibodies. As understanding increases about the immunological response in ITI, it is becoming apparent that modulation of T-cell- and B-cell-mediated responses offers a range of potential new and specific approaches to prevent and eliminate inhibitors as well as individualize ITI therapy.

The IL-10 polarized cytokine pattern in innate and adaptive immunity cells contribute to the development of FVIII inhibitors

BACKGROUND

Hemophilia A (HA) is an X-linked inherited bleeding disorder, resulting from a qualitative or quantitative deficiency of clotting factor VIII (FVIII). Antibodies against FVIII, also called inhibitors, block the procoagulant activity of FVIII; thus, impairing hemostatic activity in patients with HA. The exact mechanism underlying the immunological events behind the development of inhibitors remains unknown. This study aimed to understand immune response to FVIII in patients with HA who were either positive [HAα-FVIII(+)] or negative [HAα-FVIII(-)] for inhibitors.

METHODS

Cytokine profiles [interferon-γ (IFN - γ), tumor necrosis factor-α (TNF-α), interleukin-4 (IL-4), IL-5, and IL-10] of innate and adaptive immune cells present in the peripheral blood of participants were characterized.

RESULTS

Presence of inhibitors was significantly associated with decreased frequencies of TNF-α-positive monocytes and neutrophils, IL-5-positive monocytes, IL-4-positive neutrophils, and increased frequencies of IL-10-positive neutrophils and T cells. T cells from HAα-FVIII(-) patients expressed increased levels of almost all cytokines. In contrast, HAα-FVIII(+) patients showed lower levels of all cytokines in CD4(+) and CD8(+) T cells, except IL-10. B cells from HAα-FVIII(-) patients expressed increased levels of IL-4 while those from HAα-FVIII(+) patients expressed increased levels of IL-10.

CONCLUSIONS

The global cytokine profiles of innate and adaptive immune cells showed an anti-inflammatory/regulatory pattern in HAα-FVIII(+) patients and a mixed pattern, with a bias toward inflammatory cytokine profile, in HAα-FVIII(-) patients. The occurrence of these profiles seems to be associated with presence FVIII inhibitors.

Characterization of the anti-factor VIII immunoglobulin profile in patients with hemophilia A by use of a fluorescence-based immunoassay

BACKGROUND

The development of neutralizing antibodies, referred to as inhibitors, against factor VIII is a major complication associated with FVIII infusion therapy for the treatment of hemophilia A (HA). Previous studies have shown that a subset of HA patients and a low percentage of healthy individuals harbor non-neutralizing anti-FVIII antibodies that do not elicit the clinical manifestations associated with inhibitor development.

OBJECTIVE

To assess HA patients' anti-FVIII antibody profiles as potential predictors of clinical outcomes.

METHODS

A fluorescence immunoassay (FLI) was used to detect anti-FVIII antibodies in 491 samples from 371 HA patients.

RESULTS

Assessments of antibody profiles showed that the presence of anti-FVIII IgG1 , IgG2 or IgG4 correlated qualitatively and quantitatively with the presence of an FVIII inhibitor as determined with the Nijmegen-Bethesda assay (NBA). Forty-eight patients with a negative inhibitor history contributed serial samples to the study, including seven patients who had negative NBA titers initially and later converted to being NBA-positive. The FLI detected anti-FVIII IgG1 in five of those seven patients prior to their conversion to NBA-positive. Five of 15 serial-sample patients who had a negative inhibitor history and had anti-FVIII IgG1 later developed an inhibitor, as compared with two of 33 patients with a negative inhibitor history without anti-FVIII IgG1 .

CONCLUSIONS

These data provide a rationale for future studies designed both to monitor the dynamics of anti-FVIII antibody profiles in HA patients as a potential predictor of future inhibitor development and to assess the value of the anti-FVIII FLI as a supplement to traditional inhibitor testing.

IgG subclasses and allotypes: from structure to effector functions

Of the five immunoglobulin isotypes, immunoglobulin G (IgG) is most abundant in human serum. The four subclasses, IgG1, IgG2, IgG3, and IgG4, which are highly conserved, differ in their constant region, particularly in their hinges and upper CH2 domains. These regions are involved in binding to both IgG-Fc receptors (FcγR) and C1q. As a result, the different subclasses have different effector functions, both in terms of triggering FcγR-expressing cells, resulting in phagocytosis or antibody-dependent cell-mediated cytotoxicity, and activating complement. The Fc-regions also contain a binding epitope for the neonatal Fc receptor (FcRn), responsible for the extended half-life, placental transport, and bidirectional transport of IgG to mucosal surfaces. However, FcRn is also expressed in myeloid cells, where it participates in both phagocytosis and antigen presentation together with classical FcγR and complement. How these properties, IgG-polymorphisms and post-translational modification of the antibodies in the form of glycosylation, affect IgG-function will be the focus of the current review.

Idiopathic membranous nephropathy and IgG4: an interesting relationship

Idiopathic membranous nephropathy (iMN) is a single-organ autoimmune disease characterized by subepithelial deposition of immune complexes containing IgG4 resulting in proteinuria, nephrotic syndrome, and, in some, end-stage renal disease. The pathogenesis involves a chronic IgG4 response against specific podocyte antigens which have now been at least partially defined in the neonatal, early childhood, and adult varieties. More has recently been learned about the genetic predisposition as well. This review discusses the pathophysiology of iMN in light of these discoveries and what is known about the genesis and potential clinical ramifications of an antigen-specific IgG4 response.

A3 domain region 1803-1818 contributes to the stability of activated factor VIII and includes a binding site for activated factor IX

A recent chemical footprinting study in our laboratory suggested that region 1803-1818 might contribute to A2 domain retention in activated factor VIII (FVIIIa). This site has also been implicated to interact with activated factor IX (FIXa). Asn-1810 further comprises an N-linked glycan, which seems incompatible with a role of the amino acids 1803-1818 for FIXa or A2 domain binding. In the present study, FVIIIa stability and FIXa binding were evaluated in a FVIII-N1810C variant, and two FVIII variants in which residues 1803-1810 and 1811-1818 are replaced by the corresponding residues of factor V (FV). Enzyme kinetic studies showed that only FVIII/FV 1811-1818 has a decreased apparent binding affinity for FIXa. Flow cytometry analysis indicated that fluorescent FIXa exhibits impaired complex formation with only FVIII/FV 1811-1818 on lipospheres. Site-directed mutagenesis revealed that Phe-1816 contributes to the interaction with FIXa. To evaluate FVIIIa stability, the FVIII/FV chimeras were activated by thrombin, and the decline in cofactor function was followed over time. FVIII/FV 1803-1810 and FVIII/FV 1811-1818 but not FVIII-N1810C showed a decreased FVIIIa half-life. However, when the FVIII variants were activated in presence of FIXa, only FVIII/FV 1811-1818 demonstrated an enhanced decline in cofactor function. Surface plasmon resonance analysis revealed that the FVIII variants K1813A/K1818A, E1811A, and F1816A exhibit enhanced dissociation after activation. The results together demonstrate that the glycan at 1810 is not involved in FVIII cofactor function, and that Phe-1816 of region 1811-1818 contributes to FIXa binding. Both regions 1803-1810 and 1811-1818 contribute to FVIIIa stability.

Phenotypes of allo- and autoimmune antibody responses to FVIII characterized by surface plasmon resonance

Evidence of antibody isotype/subtype switching may provide prognostic value regarding the state of immune responses to therapeutic proteins, e.g. anti-factor VIII (FVIII) antibodies that develop in many hemophilia A patients, clinically termed “inhibitors”. A sensitive, high- information-content surface plasmon resonance (SPR) assay has been developed to quantify IgG subtype distributions and the domain specificity of anti-drug antibodies. Plasma samples from 22 subjects with an allo- or auto-immune reaction to FVIII were analyzed. Pre-analytical treatment protocols were developed to minimize non-specific binding and specific matrix interference due to von Willebrand factor-FVIII interactions. The dynamic range for IgG quantification was 0.2–5 µg/ml (∼1–33 nM), allowing characterization of inhibitor-positive samples. Subtype-specific monoclonal antibodies were used to quantify the IgG subtype distribution of FVIII-specific antibodies. Most samples obtained from multiply-infused inhibitor subjects contained IgG4 antibodies. Several distinct phenotypes were assigned based on the IgG subtype distribution: IgG₁, IgG₄, IgG₁ & IgG₄, and IgG₁, IgG₂ & IgG₄. An IgG₁-only response was found in mild/moderate HA subjects during early FVIII infusions, and analysis of serial samples followed antibody class switching as several subjects’ immune responses developed. Competition studies utilizing a recombinant FVIII-C2 domain indicated 40–80% of FVIII-specific antibodies in most samples were directed against this domain.